Circadian rhythm of rat spleen cytoplasmic thymidine kinase

Abstract

The activity of thymidine kinase (TK, EC 2.7.1.21), the first enzyme of the thymidine phosphorylation pathway, was measured at various times over a 24-hr period in the spleens of Sprague-Dawley rats that had been housed under standardized conditions of light and dark for at least 4 weeks before the study. Spleen cytoplasmic TK activity was assayed with [2- 14C]thymidine as substrate. Under “normal” light conditions (lights on 6:00 a.m.–6:00 p.m. and lights off 6:00 p.m.–6:00 a.m.), a carcadian variation of TK activity was observed (P < 0.0001, Cosinor analysis) with peak activity (1.98nmol product/hr/mg protein) at 1:00 a.m. (19 hr after light onset, HALO) and trough activity (0.40 nmol product/hr/mg protein) at 1:00 p.m. (7 HALO). Maximum enzyme activity exceeded minimum activity by approximately 5-fold. Reversing the light-dark cycle resulted in a corresponding shift in TK activity. Under these “reverse” conditions (lights on 6:00 p.m.–6:00 a.m. and lights off 6:00 a.m.–6:00 p.m.), a circadian variation in TK activity was also observed (P < 0.0001, Cosinor analysis) with peak activity (1.14nmol product/hr/mg protein) at 12:00 noon (18 HALO) and trough activity (0.32 nmol/hr/mg) protein) at 12:00 a.m. (6 HALO). Maximum enzyme activity exceeded minimum activity by approximately 4-fold. In summary, this study demonstrated for the first time that TK activity varies over a 24-hr period in association with the light-dark cycle.